Precious metal recovery from organics-precious metal compositions with supercritical water reactant

ABSTRACT

A supercritical oxidation process carried out in water is capable of oxidising “organics” in precious metal organic compositions such as heterogeneous (Pt/C) or homogeneous precious metal catalysts and producing a precious metal oxide with few by-products and low losses of precious metal.

[0001] This invention concerns improvements in precious metals refining,more especially the refining of organic-precious metal compositions.

[0002] There are very many organic-precious metal compositions whichrequire refining. These include all sorts of spent catalyst, rangingfrom heterogeneous catalysts such as a platinum group metal on a carbonsupport, e.g. 4-5% Pd on carbon, to homogeneous (liquid) catalysts suchas those based on rhodium phosphine complexes, refinery and chemicalside-streams, waste streams containing precious metals and organic,waste organo-metallic compounds and complexes and many other solids andliquids. The precious metals values contained in such compositions makeit important to recover the precious metals. Traditionally, wastescontaining precious metals and spent catalysts have been subjected toincineration. However, all such incineration processes lose significantquantities of precious metals, mostly as dusts but also possibly asvapours, and the ashes still contain very considerable quantities ofcarbon/carbonaceous material, which is difficult to remove. Thetraditional incineration processes also tend to generate rather largerquantities of pollutants, which can include NOx and dioxins and cangenerate waste water streams that are difficult to treat.

[0003] It is known that organic materials may be oxidised insupercritical water (see U.S. Pat. No. 4,338,199 for example) but we donot believe that there has been any proposal to treat organic materialscontaining precious metals. We are aware of one academic proposal totreat phenol as a waste material by supercritical water oxidation in thepresence of MnO₂ catalyst. This proposal did not suggest the presence oruse of a precious metal catalyst, and the very high cost of suchmaterials would be a disincentive. We have now discovered that a processaccording to the present invention can be carried out in a manner thatis safe, environmentally friendly, energy efficient and yields preciousmetal products in suitable form for further processing.

[0004] The organic-precious metal compositions, used as this term isused herein, used as feedstock for the process of the present inventionmay be any precious metal complex, compound or physical mixture (suchterminology is intended to include precious metals per se or preciousmetal compounds or complexes supported on an “organic” support, such asactive carbon, although carbon as such is not, strictly speaking,organic). The compositions include mixtures of precious metals orcompounds or complexes with organic materials that would otherwise beconsidered as wastes. That is, organic wastes may be admixed with aproportion of precious metal or precious metal composition and treatedaccording to the present invention, thereby oxidising the organic wasteand recovering the precious metal for further refining or othertreatment or for recycling in the treatment of more waste organic.

[0005] The present invention provides a process for the refining oforganic-precious metal compositions, comprising treating suchcomposition in a reaction fluid comprising supercritical water and asource of oxygen, permitting the organic components of the compositionto be oxidised and recovering a precious metal oxide product from thereaction products.

[0006] The supply of oxygen is conveniently done at the inlet of anelongate tube reactor, at the same time as as water is supplied to thereactor, although oxygen may alternatively or additionally be supplieddownstream of the reactor inlet, and supplementary oxygen maybe suppliedat one or more points along the length of the reactor. For example,supplementary oxygen may be fed into the reactor downstream of anorganic feedstock injection point.

[0007] The order of adding components to the reaction fluid is notespecially important. The process of the invention can be operated bypre-mixing the organic-precious metal composition and water, heating theresulting mixture to supercritical temperature, or close to thattemperature, and adding oxidant (the heat of reaction is sufficient toraise the temperature to or higher than the critical temperature). Theoxidant may also be pre-mixed with one or more of the reaction mixturecomponents.

[0008] Although it is preferred to operate a continuous process, it maybe operated as a batch process.

[0009] The quantity of oxygen used is such to achieve complete oxidationof feedstock under the reaction conditions, and is suitably adjusted byfeedback from sensors at the reactor outlet which show free oxygen andthe presence of any carbon monoxide. Desirably, the quantity is suchthat all carbon is oxidised to carbon dioxide. The oxygen is suitablysupplied from a tank of liquid oxygen. It is possible to use a mixtureof oxygen with one or more inert gases, but at present this ispreferably not used.

[0010] Although it is presently preferred to use oxygen as the oxidant,other sources of oxygen may be considered, including air, hydrogenperoxide and nitric acid.

[0011] In general, water desirably forms 90% by wt or more, for example95% by wt or more, of the entire reaction mixture.

[0012] The supercritical point of water is 374° C. and 221 bar. Anysupercritical reactor must be engineered to withstand temperature andpressures well in excess of the supercritical point. Although thematerial cost of plant to carry out the process, including special highpressure pump and valves, is intrinsically high, the fact that theprocess is simple and quick, combined with low operating costs and lowlosses of material, makes the process economical.

[0013] The process is suitably carried out at temperature in the rangefrom 400 to 600° C., preferably 500 to 580° C., and suitably at apressure from about 230 bar to about 300 bar, preferably at a pressureof 250 bar to 300 bar. There is, of course, a pressure drop across thelength of the reactor, dependent upon the individual reactor design. Thestarting temperature may, as already mentioned, be below 374° C.

[0014] The oxidation of organics according to the invention isexothermic; a temperature rise of 150° C. or so in the reactor has beenobserved 2 seconds after the injection of the feedstock, as the reactoritself operates in adiabatic mode. The reactor is preferably an elongatetube reactor, and is desirably insulated. The reactor may suitably be inthe form of a coiled tube reactor. By using conventional heatexchangers/economisers to manage heat in the various inlet and/or outletstreams it is normally possible to make the whole process operate inautothermal mode, i.e. without the addition of supplementary fuel.Indeed, a part of the heat generated in the process can be-used toproduce, for instance, high pressure steam that can be used elsewhere ona manufacturing site. The output of the process of the invention isfinely divided precious metal oxide in a supercritical aqueous slurry orsolution (before pressure let-down) which also contains the otherby-products of the oxidation reaction, dependent upon the actualchemical composition of the feedstock organic composition. Thus, if theorganic component is carbon or hydrocarbon-based, without hetero-atoms,the products are water and carbon dioxide, which creates a carbonic acidsolution after pressure let-down. If there are phosphorus, sulphur ornitrogen atoms present in the feedstock, the product contains phosphoricacid, sulphuric acid or nitrogen, respectively. It is extremelysignificant that under the reaction conditions, there is no generationof NO_(x), which is a huge advantage over conventional pyrolysis-typeprocessing. If desired, quench water may be added to the reactionproduct, and either at that point, or subsequently, it may be desirableto neutralise the acid formed by the addition of an alkali such assodium hydroxide. Generally, however, we prefer not to do so because ofthe possibility of forming insoluble salts, which may cause blockages inthe supercritical state, or the contamination of the precious metaloxide product which may complicate further processing thereof.

[0015] The precious metal component of the feedstock composition may bethe only metal present, or other metals may be present either ascomponents, e.g. as promoters of a catalyst, or as contaminants, e.g.contaminents picked up during use. In the latter case, the product ofthe process of the invention will include the highest oxidation statesof such metals. The precious metal oxide may be separated from suchcontaminent metal oxides by conventional processing that forms no partof this invention.

[0016] The invention will be further described with reference to theaccompanying schematic drawing one embodiment of a plant to carry outthe invention. Referring to the drawing, an elongate tube reactor, e.g.several hundred metres long and in the form of a coil, is generallyindicated by 1. Into the reactor is fed either two streams or a combinedstream, 2, of water and oxygen, pumped at a pressure in excess of thecritical pressure, e.g. at about 260 bar. The quantity of oxygen ispreferably adjusted by a feedback from an oxygen sensor (not shown) atthe reactor outlet for example, to achieve 10% or more O₂ by vol in theoutlet gas in a pilot plant. A full scale plant may suitably be operatedwith a lower excess of O₂. The water is fed at a rate to ensure thatthere is a high speed through the reactor to ensure that there is nosettling of solids. The organic-precious metal feedstock is fed, in theparticular pilot plant design, about ¼ of the way along the tubereactor, at which point the water is clearly supercritical. Thefeedstock itself is added at a suitable rate to give an adiabatictemperature rise. The feedstock may be a liquid organic-precious metalcomposition, in which case it is pumped directly into the reactor, ormay be solid, in which case it is slurried in water before being pumpedinto the reactor. The output from the reactor may be diluted and/orneutralised with a quench water stream shown by, 3, although this isoptional, before passing through suitable pressure let down equipment,4, (shown as a valve) before passing into a gas/liquid separation tank,5. Excess gases, e.g. O₂, N₂, CO₂ etc are taken off through line, 6, anda slurry of precious metal oxide is taken through line, 7, at the baseof the separation tank. The precious metal oxide tends to be in veryfinely divided form if the feedstock is liquid, and in particlescorresponding to the form of the feedstock metal if solid, e.g. if a Pdon carbon catalyst. The solids may then be separated and subjected tosuch further treatment or refining as is necessary or desirable.

[0017] In order to economise on water usage, it is preferred to recyclewater. Similarly, good engineering design provides heat exchange betweenstreams.

[0018] It should be understood that the drawing and the above specificdescription, relates to a “direct injection” mode of operation. Theskilled person will understand that the invention may be operated in anumber of ways differing in detail. For example, and especially forsolid feedstocks such as Pd/C or Pt/C, the feedstock may desirably beslurried in water, fed via a heat exchanger to the reactor and thenoxygen is injected to carry out the oxidation reactions.

[0019] The invention will now be described by way of working examples.

EXAMPLE 1

[0020] 80 Kg of spent Pt on carbon catalyst was slurried with 720 Kg ofwater, passed through a grinder pump to reduce particle size and fed toa supercritical water oxidiser reactor at a rate of 250 Kg/hr. The freshcatalyst was 5 wt % Pt on carbon, but the spent catalyst was assayed at1.6 wt % Pt on carbon. Oxygen was fed to the reactor at a rate adjustedto yield 15% O₂ in the output gas. The output slurry was filtered toyield a fine, black Pt oxide and a clear, light yellow filtrate. Thefiltrate contained less than 0.5 ppm Pt.

EXAMPLE 2

[0021] The process of Example 1 was used to treat an unused 5 wt %Pd/carbon catalyst doped with toluene. The filtrate contained less than0.01 ppm Pd. Typically, a spent catalyst contains 0.8 wt % Pd on carbon,and can also be treated according to the invention.

EXAMPLE 3

[0022] The process of Example 1 was adapted to feed a rhodium phosphineliquid organic catalyst stream, containing 0.973% Rh by weight directlyonto the reactor. The product liquor was a slurry of very fine blackparticles in a colourless solution.

[0023] In all the above Examples, the recovery of precious metal anddestruction of organic was well in excess of 95% by wt. The process maybe expected to yield even better results upon optimisation.

1. A process for the refining of organic-precious metal compositions,comprising treating such composition in a reaction fluid comprisingsupercritical water and a source of oxygen, permitting at least theorganic components of the composition to be oxidised and recovering aprecious metal oxide from the reaction products.
 2. A process accordingto claim 1, wherein the organic precious metal composition comprises oneor more of the platinum group metals.
 3. A process according to claim 1or 2, wherein the source of oxygen is oxygen.
 4. A process according toclaim 3, wherein the oxygen is supplied from liquid oxygen.
 5. A processaccording to any one of the preceding claims, carried out at atemperature of from 400 to 600° C.
 6. A process according to any one ofthe preceding claims, carried out in a continuous elongate reactor.
 7. Aprocess according to claim 6, wherein organic-precious metal compositionis admixed with water and fed to the reactor, and oxygen is subsequentlyadded.
 8. A process according to claim 7, wherein the mixture is fed ata temperature below the critical temperature of water into the reactor,and the exotherm from the oxidation provides supercritical temperatures.